1. Field of the Invention
The present invention relates generally to an actively controlled suspension system for an automotive vehicle, which controls suspension characteristics depending upon vehicle driving condition in order to suppress attitude change of a vehicular body. More specifically, the invention relates to a fail-safe system in the actively controlled suspension system, which performs fail-safe operation in response to failure of components of the suspension control system, particularly electric circuitry.
2. Description of the Background Art
U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987 which has been assigned to the common owner to the present invention, discloses one of typical construction of an actively controlled suspension system, in which a hydraulic cylinder defining a working chamber is disposed between a vehicular body and a suspension member rotatably supporting a vehicular wheel. The working chamber of the hydraulic cylinder is communicated with a hydraulic circuit including a pressurized working fluid source. A pressure control valve, such as an proportioning valve assembly, is disposed in the hydraulic circuit, which is connected to an electric or electronic control circuit to be controlled the valve position. The pressure control valve is controlled the valve position by a suspension control signal produced in the control circuit for adjusting pressure in the working chamber and whereby controlling suspension characteristics.
On the other hand, European Patent Nos. 0 283 004, 0 285 153 and 0 284 053 discloses technologies for controlling the suspension systems constructed as set forth above, depending upon the vehicle driving condition for suppressing rolling and/or pitching of the vehicular body.
As will be appreciated, for the automotive suspension systems, very high security has been required for assure safety in driving. Therefore, for the actively controlled suspension systems set forth above, it is important to assure safety at any vehicular driving condition. Furthermore, security is required for such electrically or electronically controlled suspension system even when one of the components of the suspension control system fails.